The present invention relates to a power transmission belt or drive belt which has good heat resistance and low temperature resistance, together with high reliability.
Power transmission belts are widely used in automobiles and for general industrial use, among others, as frictional belts for power transmission, such as a wrapped molded V-belt, a raw-edged V-belt and like V belts and V-ribbed belts, and further as engaging belts for power transmission, such as timing belts.
Such a power transmission belt is generally constituted of an adhesive rubber layer with core wires embedded therein, and a bottom rubber layer overlaid with said adhesive rubber layer, if necessary together with an upper canvas and/or a lower canvas as adhered to the respective layers.
It is thought that the most effective measure for prolonging the life of a power transmission belt is to improve the same with respect to the life thereof in a high temperature atmosphere in which rubber degradation is markedly accelerated. To cope with the conditions of environment under which it is used, a rubber species showing good flexibility at low temperatures is demanded.
So far, natural rubber (NR), styrene/butadiene copolymer rubber (SBR), chloroprene rubber (CR), hydrogenated nitrile/butadiene rubber (H-NBR) and alkylated chlorosulfonated polyethylene (ACSM) have been widely used as the rubber constituting the adhesive layer and bottom rubber layer of such a power transmission belt.
Among such rubbers, natural rubber and styrene/butadiene copolymer rubber have good low temperature resistance but are inferior in heat resistance to chloroprene rubber. On the other hand, hydrogenated nitrile/butadiene rubber and alkylated chlorosulfonated polyethylene have good heat resistance but are inferior in low temperature resistance to chloroprene rubber. Thus, chloroprene rubber is superior in both heat resistance and low temperature resistance to the other rubbers mentioned above and therefore is generally used as the rubber for power transmission belts.
As a rubber superior in both heat resistance and low temperature resistance to chloroprene rubber, there may be mentioned, for example, epichlorohydrin rubber. However, when this rubber is used in belts, a problem arises; namely, slipping occurs on pulleys and this slipping causes the rubber to soften, whereupon a tacky matter forms in large quantities.
Meanwhile, Japanese Kohyo Publication Hei-09-500930 discloses a belt the rubber component of which is a cured composition comprising ethylene/propylene copolymer rubber (EPM) and/or an ethylene/propylene/diene terpolymer (EPDM), an xcex1, xcex2unsaturated carboxylic acid salt, an organic peroxide and other additives.
Such ethylene/propylene/diene termpolymer is superior in heat resistance and low temperature resistance to chloroprene rubber and, furthermore, is characterized in that even when slipping occurs, the rubber itself will not soften; hence the life of said belt is long.
Thus, the conventional belt produced by using an ethylene/propylene/diene terpolymer is excellent in the heat resistance in a range of high temperature where the degradation of rubber dominates the life of the belt and, at the same time, is almost comparable in life span to chloroprene rubber even in the temperature range of 20xc2x0 C. to 80 xc2x0 C. in which the flexural fatigue of rubber determines the life of the belt. In the ordinary mode of use in which the power transmission belt is most frequently used at about 20xc2x0 C. to 80xc2x0 C., the life of a belt comprising said terpolymer does not differ from that of a chloroprene rubber belt barring exceptionally severe conditions.
However, the ethylene/propylene/diene terpolymer belt has problems; once a tiny crack has appeared in the production process, during transportation or in the step of mounting, for instance, the crack immediately widens, leading to belt destruction. Thus, early failure has occurred frequently.
During use thereof as well, when an impact is given to said belt, for example by inclusion of a foreign matter between the belt and a pulley or by a rusted pulley, cracking tends to occur. Once formed, a crack immediately progresses, leading to destruction of the belt, so that said belt is lacking in reliability as compared with chloroprene rubber which is slow in the progression of cracks.
In view of the foregoing, the present invention has for its object to provide a power transmission belt having good heat resistance and low temperature resistance and showing only slight variation in life owing to slow crack development and, further, showing high reliability.
The present invention consists in a power transmission belt the main constituent rubber composition of which comprises not less than 50% by weight, on the total polymer component basis, of an ethylene/xcex1-olefin/diene terpolymer with sulfur incorporated therein as a curing or vulcanizing agent.